The present invention relates to fluid loss control for drill-in fluids for use in subterranean formations, and more particularly to fluid loss control for drill-in fluids and filter cakes comprising degradable bridging agents, and to methods of using such fluids in subterranean drilling operations.
Many oil and gas wells in unconsolidated or poorly consolidated sandstone formations are completed “open hole,” meaning that the well bores do not contain casing or liners. Although this type of completion allows the produced fluids to flow directly into the well bore, it suffers the disadvantage that the sandface is unsupported and may collapse. Also, selective treatments or remedial operations within the reservoir section may be more difficult.
Unconsolidated or poorly consolidated formations generally are high permeability production intervals and are drilled with specialized fluids referred to in the art as “drill-in fluids.” A drill-in fluid generally comprises two components: particulate solids (e.g., for bridging on the pore throats of the sandstone of the formation); and polymeric components (e.g., for providing viscosity and fluid loss control). Under pressurized downhole conditions, the drill-in fluid may form a filter cake that comprises an acid-soluble portion (e.g., calcium carbonate bridging solids) and a polymeric portion on the face of a portion of the subterranean formation. In most instances, once formed, the integrity of the filter cake should be maintained to provide the necessary fluid loss control and hole stability for subsequent operations. A common subsequent treatment is a gravel pack sand control operation that involves running a screen into the open hole interval, and pumping a gravel pack fluid comprising gravel into the annulus between the screen and open hole to form a gravel pack.
Generally, at some point after the gravel pack is placed, it is desirable to remove the filter cake from the formation face as it may act as an impediment to the production of desirable fluids from the formation. However, degrading the filter cake may be difficult since the screen and gravel pack tend to prevent the components designed to degrade the filter cake from interacting with it. Degrading the filter cake may be even more difficult, considering that the degradation is generally best when it is uniformly accomplished along what may be thousands of feet of open hole. Thus, because the gravel and gravel pack carrier fluid contact the filter cake uniformly across the entire interval, placing components with the gravel pack that are capable of ultimately degrading the filter cake would be desirable if such degradation could be delayed long enough to ensure that the placement of the gravel pack treatment is not jeopardized or high fluid loss rates are not incurred until the completion equipment is installed.
To degrade the acid-soluble particulate portion of the drill-in fluid filter cake, acids or delayed-release acid systems may be used. A common type of delayed-release acid system comprises acid precursors that slowly hydrolyze to form acids that may ultimately degrade the acid-soluble portion of the filter cake. These delayed-release acid systems, however, can be problematic if they degrade the acid-soluble component of the filter cake too slowly or too quickly. Removal of only 1% to 2% of the bridging solids in the filter cake can result in a significant loss of fluid to the surrounding formation. If a delayed-release acid system is designed not to dissolve more than 1% or 2% of the acid-soluble portion of the filter cake in a chosen period of time (e.g., a 12-hour period), then total removal may take days, if not weeks. This is undesirable. On the other hand, if a delayed-release acid system is designed to totally degrade the acid-soluble portion within an acceptable “total cleanup time” (e.g., 24 to 48 hours), it is likely to cause hole instability and potential fluid loss problems during gravel pack placement. To control such fast-acting delayed-release acid systems, buffers (which are mixtures of weak acids and their conjugate bases) may be considered to achieve a delayed interaction of the acid with the acid-soluble portion of the filter cake for a desired time period. However, such buffer systems have met with little success when used with these delayed-release acid systems, inter alia, because the esters may undergo acid- or base-catalyzed hydrolysis at pHs much below or above 7. Also, conventional buffers may suffer when exposed to components, such as calcium carbonate, in the filter cake and, as a result, the acid component of the buffer may be quickly consumed.
Oxidizers have been used to degrade the polymeric portions of filter cakes within desired delay and total cleanup times. Since these oxidizers are not able to degrade the acid-soluble portion of a filter cake, the usefulness of such oxidizer systems generally is limited to cases where the bridging particles that comprise the particulate portion of the filter cake are small enough to flow back through the screen.
High permeability sandstone can be problematic for drill-in fluids because of fluid loss into the formation. Fluid loss is undesirable as it results in more fluid needing to be pumped, which increases expense. Moreover, the fluid that leaks off into the formation can cause damage to the formation, which may decrease permeability and/or productivity.